1. Field of the Invention
This invention relates to a copper base alloy that is particularly suited to be formed into electrical connectors. The copper base alloy contains iron, phosphorous and zinc to which magnesium is added within certain limits. The alloy provides improved stress relaxation resistance at elevated temperatures. The alloy also provides an excellent combination of properties including high electrical conductivity, excellent bend formability and high strength.
2. Description of Related Art
Alloys of copper with iron, zinc and phosphorus, represent an important group of high copper alloys (defined as having 96% minimum copper). High copper alloys are used for a broad range of applications that require moderate to high electrical conductivity in combination with high strength and adequate formability. An important use for this group of copper alloys is for the spring contact member in electrical connectors. High contact force, and associated low contact resistance, is attainable because of these alloys"" strength. The good electrical conductivity typical of these alloys also permits management of large electrical currents without unacceptable resistance heating.
However, high copper alloys of the group that further includes iron, zinc and phosphorous are typically limited to service temperatures below around 100xc2x0 C. (212xc2x0 F.) because of limited resistance to losses in contact force during prolonged thermal exposure, a phenomenon referred to as stress relaxation.
One high copper alloy used to manufacture electrical connector components is designated by the Copper Development Association (xe2x80x9cCDAxe2x80x9d, New York, N.Y.) as copper alloy C19400. Copper alloy C19400 has a composition specified by CDA, by weight, 2.1%-2.6% iron, 0.05%-0.20% zinc, 0.015%-0.15% phosphorous, and the balance copper and unavoidable impurities. Alloys of this type are disclosed in U.S. Pat. Nos. 3,039,867, 3,522,039 and 3,522,112 to C. D. McLain, all of which are incorporated by reference in their entireties herein. Copper alloy C19400 has been utilized for lead materials, such as leadframes, as well as for connector applications.
Various attempts have been made to modify copper alloy C19400 through the addition of other elements, such as aluminum, silicon, manganese or magnesium, in small amounts. U.S. Pat. Nos. 3,522,038 to C. D. McLain, 3,671,225 to C. D. McLain, 3,671,225 to C. D. McLain and 4,668,471 to Futatsuka et al. are illustrative of such attempts. These four U.S. patents are also incorporated by reference in their entireties herein.
The Futatsuka patent relates to a copper alloy lead material for leads in semiconductor devices. The alloy is comprised of 2-2.4 wt. % iron, 0.001-0.1 wt. % phosphorous, 0.01-1 wt. % zinc, 0.001 to 0.1 wt. % magnesium and the balance copper and inevitable impurities. The patent recites that magnesium improves strength, heat resistance and soldering reliability of the material for leads without sacrificing the elongation and conductivity of the alloy. Heat resistance refers to the ability of an alloy to resist softening due to recovery and recrystallization upon exposure to elevated temperatures in the absence of an externally applied stress. Heat resistance is distinguished from stress relaxation resistance which is the ability of an alloy to maintain its spring force in use at temperatures below its recrystallization temperature.
The Futatsuka patent limits the upper limit of the magnesium addition to 0.1 wt. % and states that if the magnesium content exceeds that level the lead material will have degraded electrical conductivity and the molten alloy will have degraded fluidity, thus making casting of the alloy difficult.
Magnesium has been proposed for use in a number of high copper alloys. U.S. Pat. No. 3,698,965 discloses an alloy having 0.2-4.0 wt. % iron, 0.10-1.0 wt. % of a material selected from magnesium, tin and mixtures thereof, 0.01-0.5 wt. % phosphorous, 0.2-2.5 wt. % cobalt, with iron plus cobalt being between 1-5 wt. % and the remainder copper. U.S. Pat. No. 4,605,532 discloses a copper alloy containing 0.3-1.6 wt. % iron, with up to one-half the iron content being replaced by nickel, manganese, cobalt, and mixtures thereof, 0.01-0.20 wt. % magnesium, 0.10-0.40 wt. % phosphorous, up to about 0.5 wt. % tin or antimony or mixtures thereof and the balance copper. In this alloy the phosphorous to magnesium ratio and the phosphorus to the total content of phosphide formers ratio are maintained within critical limits. U.S. Pat. No. 5,868,877 discloses a copper alloy having 0.1-0.17 wt. % phosphorous, 0.1-1.5 wt. % iron and the balance is copper and unavoidable impurities. The ""877 patent discloses that the alloy requires free magnesium in solid solution in accordance with a specific formula to improve stress relaxation resistance.
In accordance with the ""877 patent, it is understood that xe2x80x9cfree magnesiumxe2x80x9d refers to magnesium in solution with copper as opposed to a form that precipitates from the alloy matrix during processing.
The maximum iron content, however, claimed in the ""532 and ""877 patents is 1.6% and 1.5%, respectively and the minimum phosphorus content is 0.1% in both patents. In the ""965 patent, magnesium is consumed by the tin as an intermetallic phase. These high copper alloys are separate and distinct alloys as compared to the copper alloys of this invention and the effects of magnesium in these alloys do not provide predictability of the effect of magnesium in the inventive alloys. These patents are incorporated by reference in their entireties herein.
Other patents that disclose high copper alloys containing iron, phosphorous and magnesium with even lower iron contents include U.S. Pat. Nos. 4,305,762 and 4,605,532 and published Japanese Patent Application No. JP 58-199835.
JP11-264037 discloses a foil formed from a copper alloy that contains, by weight, 0.05%-3.5% iron and 0.01%-0.4% phosphorous. Optionally the alloy may contain one or both of 0.05%-5% zinc and 0.05%-3% tin. The alloy may further contain one or more of Mg, Co, Pb, Zn, Cr, Mn, Al, Ni, Si, In and B in an amount of 0.01%-2% in total.
Modern electronic connector applications require materials which exhibit excellent stress relaxation resistance when exposed to elevated temperature environments in order to insure sustained reliable electrical contact. For example, in automotive environments an electrical connector in the engine compartment can be exposed to operating temperatures above 100xc2x0 C. Improvement in the stress relaxation resistance of high copper alloys is needed to meet the increased requirements posed by such modern connector applications.
The design of electrical/electronic connectors, particularly for use in the automotive industry, has become much more complex and miniaturized. This has imposed increasingly higher stress relaxation demands on the copper alloys from which they are made. This invention concerns improving the resistance to stress relaxation of a copper-iron-phosphorus-zinc alloy by a controlled addition of magnesium, while maintaining a good combination of strength, electrical conductivity and formability. The alloy of this invention provides an excellent combination of properties including good bend formability, high strength and improved resistance to stress relaxation at elevated temperatures.
In accordance with this invention, a copper alloy is provided having improved resistance to stress relaxation. The alloy consist essentially of: from about 1.8 to 3.0 weight percent iron; from about 0.01 to about 1.0 weight percent zinc; from about 0.001 to about 0.25 weight percent phosphorous; from greater than about 0.1 to about 0.35 weight percent magnesium; and the balance copper and unavoidable impurities.
Preferably the copper alloy includes: iron from about 2.0 to 2.7 weight percent; zinc from about 0.01 to about 0.5 weight percent; phosphorous from about 0.010 to about 0.15 weight percent; and magnesium from about 0.11 to about 0.30 weight percent.
Most preferably, the copper alloy includes: iron from about 2.1 to 2.6 weight percent; zinc from about 0.05 to about 0.25 weight percent; phosphorous from about 0.01 to about 0.09 weight percent; and magnesium from about 0.15 to about 0.25 weight percent.
Optionally, cobalt may be substituted, in whole or in part, on a 1:1 basis by weight, for iron.
The copper alloy in the stress relief annealed condition preferably has a yield strength of from 45 to 80 ksi, an electrical conductivity of greater than or equal to 60% IACS, stress relaxation resistance at 150xc2x0 centigrade of at least 70% longitudinal stress remaining after 3000 hours exposure and good bend formability.
IACS refers to International Annealed Copper Standard that assigns a conductivity value of 100% to xe2x80x9cpurexe2x80x9d copper at 20xc2x0 C.
Preferably the alloy of the invention is in a stress relief annealed condition and is substantially free of magnesium phosphides. The preferred use of the alloy of this invention is for electrical/electronic connector applications, although the alloy may be used in any application where its unique combination of properties makes it suitable, such as without limitation, leadframes or other electronic uses, wires, rods and foil.
A process for making a copper alloy in accordance with this invention also forms part of the invention. An electrical connector formed from the copper alloy of this invention also forms part of this invention.
Accordingly, it is an aim of the present invention to provide an improved copper base alloy and the process for making it, which will provide an alloy having increased stress relaxation resistance.
It is a further aim of this invention to provide a high copper alloy to which magnesium is added within certain limits.
It is a still further aim of this invention, in accordance with a preferred embodiment thereof, to provide an alloy which has an excellent combination of properties including good bend formability, high strength, excellent stampability and improved resistance to stress relaxation at elevated temperatures.
The above stated objects, features and advantages will become more apparent from the specification and the drawings that follows.